1. Field of the Invention
The present invention relates to a transfer drum of an image forming apparatus used when a toner image formed on a photoconductive drum of an image forming apparatus such as a copying machine and a printer is transferred to a transfer material such as a recording sheet.
2. Description of the Related Art
One of conventional transfer drums is shown in FIGS. 12 and 13. In FIG. 13, the cylindrical photosensitive portions 12 is provided at a center portion of the photosensitive drum 101 along the shaft direction. The photosensitive gear 103 is provided on one end portion of the shaft direction (left end portion in FIG. 13).
The transfer drum 106 is used to transfer the toner image formed on the cylindrical photosensitive portion 102 of this photosensitive drum 101 to the transfer material. The transfer drum 106 is so arranged that this transfer drum 106 is movable between a position (saving position) apart from the photosensitive drum 101 and another position (operating position) close thereto.
The transfer roller 106 includes a front drum 107, a rear drum 108, and a drum tie-plate 109 for coupling the front and rear drums 107 and 108. They are made of aluminium. Cylindrical members 111 and 112 for supporting bearings are fixed at center portions of both drums 107 and 108. A shaft member 113 is supported on the cylindrical members 111 and 112 via a bearing (not shown). This shaft member 113 are extended outwardly from both drums 106 and 107 (see FIG. 12) and is supported by a frame (not shown) of an image forming apparatus.
A transfer film 114 (see FIG. 13) is spread in a cylindrical shape between both drums 107 and 108, and then a transfer material such as paper and a recording sheet (not shown) adheres to this cylindrical transfer film 114 by means of static electricity.
In FIGS. 12 and 13, a ring-shaped driven gear 116 is mounted on the left end portion of the front drum 107 of the transfer drum 106. The ring-shaped driven gear 116 meshes with a photosensitive side gear 103 of the photosensitive drum 101. A ring-shaped rubber belt 117 is mounted on the right side of this transfer-sided gear 116. As shown in FIG. 13, this rubber belt 117 is attached to the cylindrical surface of the photosensitive drum 101, so that an interval (DDS dimension, see FIG. 13) between the surface of the photosensitive drum portion 102 and the surface of the transfer film 114 is maintained at a predetermined value.
Although the transfer-sided gear is not provided with the rear drum 108, a rubber belt 118 (see FIG. 12) similar to the ring-shaped rubber belt 117 is provided therewith.
A tolerance of an outer diameter required for the transfer drum 106 is selected with in the range of .+-.0.02 mm in a monochromatic toner image forming apparatus, namely rather high precision. Therefore, conventionally, the front drum 107, rear drum 108, drum tie-plate 109, and transfer-sided gear 116 and the like are made of aluminium, and are processed by cutting machines in order to ensure the dimensional precision.
As a result, the number of working steps for mounting the transfer-sided gear 116 on the front drum 107 and for cutting the above-described members has to be increased, and therefore it is rather difficult to lower the manufacturing cost.
Further, Unexamined Japanese Patent Publication No. Sho. 58-50564 discloses the method for manufacturing all of the front drum, rear drum, and drum tie-plate by way of a resin integral form (molding).
However, the method described in Unexamined Japanese Patent Publication No. Sho. 58-50564 has the following problem. Generally, as described in the above publication, it is well known that the material of the molding member is changed from the metal material into the resin material. However, in the case that either strength or precision is required for the molding member, a certain improvement should be required. When the overall transfer drum is molded by the resin as disclosed in the publication, in such a transfer apparatus that a transfer film is spread on a transfer drum, it is difficult to ensure precision in alignments of a front drum and rear drum which constitute important dimension in an image forming apparatus. Also, mechanical strength such as deflection and torsion is also important in view of the structure of this transfer drum. As to the molding members requiring such high precision and high strength, it is practically not possible to ensure such requirements even by employing any types of resin materials. In practically, it is impossible to manufacture the entire transfer drum by using a resin.
Except the idea disclosed in the publication, conventionally, the transfer drums having the structures indicated in FIGS. 12 and 13 is practically used. To ensure the desired high precision of one pair of drums for constituting the transfer drum, the molding product of aluminium is processed by the cutting tool. This type of practically available transfer drum would require high cost by performing the cutting work.
In the drum having the transfer-sided gear and the transfer-sided positioning cylinder surface among one pair of drums (front drum and rear drum) for constituting the transfer drum, the drum base member corresponding to this constructive element, the transfer-sided gear mounted on the outer peripheral portion, the ring-shaped rubber belt for constructing the transfer-sided positioning cylindrical surface, and the sensor shielding plate are separately constructed. The reason why this drum is constructed of the separate members is to ensure the precision. Unless such a separate construction is employed, it is conceived that the desired precision would not be easily obtained.
The drum having the transfer-sided gear and the transfer-sided positioning cylinder surface at the outer peripheral portion would require the work to manufacture the respective elements (drum base member, transfer-sided gear, ring-shaped rubber, and sensor shielding plate) separately constructed, and the work to assemble these separate elements, resulting in high cost.